Sol-Gel Prepared NiO...
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136 Acta Chim. Slov . 2006, 53, 136–147 Cerc Korošec and Bukovec Sol-Gel Prepared NiO Thin Films for.... Review Paper Sol-Gel Prepared NiO Thin Films for Electrochromic Applications Romana Cerc Korošec and Peter Bukovec F aculty of Chemi stry and Chemica l T echology , University of Ljubljana, Aškerčeva 5, SI-1000 Ljubljana, Slovenia e-mail: [email protected] Received 19-04-2006 Abstract An electrochromic material changes its optical properties in the visible part of the spectrum under a certain applied potential. The change is reversible and the material returns to its original state under the opposite electric field. Recently, electrochromism has been applied in electrochromic devices, where in a battery-like assembly the throughput of solar light is controlled by the voltage and is usually termed a smart window. In the first part of this article a brief theoretical introduction to electrochromism and the functioning of smart windows is given. Since in the last decade nickel o xide has been extensively studied as an ion-storage material in electrochromic devices, some properties of nickel oxide are explained in the following part. The electrochromic response (reversibility during potential switching and the degree of coloration) of a nickel oxide thin film, used in a electrochromic device, strongly depends on the degree of heat treatment. Thermal analysis of thin films can give valuable information about a suitable temperature and the duration of heat-treatment when thin films are prepared by chemical methods of deposition. Since thermal analysis of thin films deposited on a substrate is not a common analytical technique, basic strategies are also summarized in the article. After this theoretical introduction, the application of TG analysis to optimisation of the electrochromic response of sol-gel prepared Ni oxide thin films is presented. The electrochromic properties of thin films, thermally treated to different degrees, were tested using spectroelectrochemical methods. Additional techniques (IR, TEM, AFM and EXAFS) were indispensable in following structural and morphological changes during the heat treatment. Keywords: electrochromism, NiO thin films, thermal analysis, TG, sol-gel, optimisation 1. Electrochromism, elect rochromic materials (ECMs) and electrochromic devices (ECDs) ...................................................................136 2. lectrochromic nickel oxide t hin films and the proposed coloration mechanism ............................................................138 3. Thermal analysi s of thin films ............................................................................139 4. Sol-g el pre pare d Ni oxi de thi n films from NiSO 4 precursor ..............................140 5. Sol-g el pre pare d Ni oxi de thi n films from Ni(CH 3 COO) 2 precursor .................143 6. Conclusion .........................................................................................................145 7. References .........................................................................................................146 Contents 1. Electrochromism, electrochromic materials (ECMs) and electrochromic devices (ECDs) By definition an electrochromic material is able to reversibly and persistently change its optical properties under an applied electric field. 1 Electrochromic materials came to public attention around 30 years ago after the report on the electrochromism of tungsten oxide by Deb. 2 Up to now, several different applications of electrochromic devices were developed, but for some of them – for information displays, for instance - commercialization has not taken place. Development was mor e suc cess ful in the cas e of e lect roch romic automatically dimming rear-view mirrors, which are now generally available for cars and trucks 3 and for smart
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